(1) Field of the Invention
The present invention relates to signal transmission systems and, more particularly, to a signal transmission system for transmitting an alarm signal or a control signal.
(2) Description of the Related Art
In a transmission system in a subscriber communications service in which system an analog telephone line, an ISDN line and a dedicated line coexist, a remote alarm signal for testing each line and a remote control signal need to be transmitted apart from a main signal.
FIG. 1 shows an example of conventional system for transmitting signals such as an alarm signal or a control signal from a transmitter 31 to a receiver 32. In such a conventional system, the transmitter 31 receives an alarm signal from an alarm source, or receives a control signal to a target of control, the signals being received as transmitter input signals. The transmitter 31 transmits these signals to the receiver 32 via a transmission channel 33.
Either the transmitter 31 forwards the transmitter input signals unmodified to the receiver 32, or the level of these signals is stabilized by means of a protection circuit formed by a pull-up resistance 34 or a capacitor 35 so as to prevent errors in the transmitted signals from occurring when a failure takes place in the transmitter 31 or the transmission channel 33.
In such a conventional system, it is difficult to prevent errors in the transmitter signals from occurring only by a protection circuit formed of a pull-up resistance or a capacitor. While such an arrangement will not present a problem when the information sent is not vulnerable to errors, the reliability of the transmission system will fall in the case of the information vulnerable to errors.